One existing type of switch is a radio frequency (RF) micro-electro-mechanical switch (MEMS). This existing type of switch has a substrate with two spaced and conductive posts thereon. A conductive part is provided on the substrate between the posts, and is covered by a layer of a dielectric material. A flexible and electrically conductive membrane extends between the posts, so that a central portion of the membrane is located above the conductive part on the substrate. An RF signal is applied to one of the conductive part and the membrane.
In the deactuated state of the switch, the membrane is spaced above both the conductive part and the dielectric layer covering it. In order to actuate the switch, a direct current (DC) bias voltage is applied between the membrane and the conductive part. This bias voltage produces charges on the membrane and the conductive part, and the charges cause the membrane and conductive part to be electrostatically attracted to each other. This attraction causes the membrane to flex, so that a central portion thereof moves downwardly until it contacts the top of the dielectric layer on the conductive part. This is the actuated position of the membrane.
In this actuated state of the switch, the spacing between the membrane and the conductive part is less than in the deactuated state. Therefore, in the actuated state, the capacitive coupling between the membrane and the conductive part is significantly larger than in the deactuated state. Consequently, in the actuated state, the RF signal traveling through one of the membrane and conductive part is capacitively coupled substantially in its entirety to the other thereof.
In order to deactuate the switch, the DC bias voltage is turned off. The inherent resilience of the membrane then returns the membrane to its original position, which represents the deactuated state of the switch. Because the capacitive coupling between the membrane and conductive part is much lower in the deactuated state, the RF signal traveling through one of the membrane and capacitive part experiences little or no capacitive coupling to the other thereof.
Although existing switches of this type have been generally adequate for their intended purposes, they have not been satisfactory in all respects. One problem is that, when the membrane is contacting the dielectric layer in the actuated state of the switch, electric charge from the membrane can tunnel into and become trapped in the dielectric layer. As a result, and due to long recombination times in the dielectric, the amount of this trapped charge in the dielectric increases progressively over time.
The progressively increasing amount of trapped charge exerts a progressively increasing attractive force on the membrane. When the membrane is in its actuated position, this attractive force tends to resist movement of the membrane away from its actuated position toward its deactuated position. The amount of trapped charge can eventually increase to the point where the attractive force exerted on the membrane by the trapped charge is in excess of the inherent resilient force of the membrane which is urging the membrane to return to its deactuated position. As a result, the membrane becomes trapped in its actuated position, and the switch is no longer capable of carrying out a switching function. This is considered a failure of the switch, and is associated with an undesirably short operational lifetime for the switch. In this regard, an RF MEMS switch of this type should be capable of trillions of switching cycles before a failure occurs due to fatigue in the metal of the membrane, but trapped charge in the dielectric usually results in failure after only millions of switching cycles.
There are many applications in which a switching function can be implemented using either a field effect transistor (FET) switch or an RF MEMS switch. However, due in significant part to the dielectric charging problem discussed above, the operational lifetime of existing MEMS switches is significantly shorter than the operational lifetime of commercially available FET switches. Consequently, FET switches are currently favored over MEMS switches for these applications.
Prior attempts have been made to solve the dielectric charging problem. One approach was to change the properties of the dielectric material so as to modify the extent to which the dielectric material is “leaky”. For example, by adding more silicon to silicon nitride used for the dielectric material, the conductivity of the dielectric material increases, and then it becomes easier for the trapped charges to recombine in a manner which neutralizes them. However, this approach also increases the power consumption of the MEMS switch, and has not been shown to provide a significant increase in its operational lifetime.
Another prior approach to the dielectric charging problem is to alter the waveform used for the DC bias voltage. For example, lowering the actuation voltage reduces the amount of charge which tunnels into the dielectric material, and thus reduces the rate at which the amount of trapped charge within the dielectric material can increase. Further, the slope of the release waveform can be decreased, so as to give the trapped charges more time to recombine. These types of changes to the actuation waveform can produce a significant increase in the operational lifetime of a MEMS switch. However, they also significantly increase the switching time of the switch, for example by a factor of approximately 20, which in turn renders such a MEMS switch highly undesirable for many applications that involve high switching speeds.
In the design of MEMS switches, a traditional design goal has been to try to maximize the capacitance ratio of the switch, which is the ratio of the capacitance between the membrane and conductive part in the actuated state to the corresponding capacitance in the deactuated state. In an effort to maximize the capacitance in the actuated state, pre-existing MEMS switch designs attempt to position the membrane as close as possible to the conductive part in the actuated state of the switch, which in turn means that the dielectric layer separating them needs to be relatively thin. Consequently, the surfaces of the membrane and dielectric layer which engage each other have traditionally been intentionally polished or otherwise fabricated to make them as smooth as possible, so that both surfaces have their entire areas in direct physical contact with each other when the membrane is in its actuated position, thereby positioning as much of the membrane as possible in very close proximity to the conductive part.